1. Field of the Invention
The present invention relates to a receiving device and an integrated circuit for reception.
2. Description of the Related Art
Digital audio radio services in the U.S. are called “DARS”, and in DARS, satellite waves and terrestrial waves are used in combination so that even a receiver mounted in a mobile unit such as vehicle can reliably receive the radio waves.
More specifically, in the DARS, a 2.3 GHz band is used, and as shown in part B of FIG. 6, two services are broadcast. Currently, each of the services uses a frequency band of 12.5 MHz. As is also shown in part A of FIG. 6, one service is formed of two ensembles A and B, and each of these ensembles A and B provides 50 channels of programs contents. Therefore, one service provides programs of 100 channels.
The ensemble A is broadcast with individual signals A1, A2, and A3, and the ensemble B is broadcast with individual signals B1, B2, and B3. That is, the contents of the signals A1, A2, and A3 are the same, and the contents of the signals B1, B2, and B3 are the same. Therefore, if any one of the signals A1, A2, and A3 can be received, the program of the ensemble A can be listened to, and in a similar manner, if any one of the signals B1, B2, and B3 can be received, the program of the ensemble B can be listened to.
As is also shown in part A of FIG. 6, the signals A1 to A3 and B1 to B3 are arranged as the signals A1, A2, A3, B3, B2, and B1 in order of frequency, and the signals A1, A2, and A3, and the signals B3, B2, and B1 are symmetrically placed about a center frequency fC between the signal A3 and the signal B3.
The signals A1, A2, B1, and B2 are QPSK (Quadrature Phase Shift Keying) signals. The signals A1 and B1 are transmitted from a broadcasting satellite BS1 over the Western U.S., and the signals A2 and B2 are transmitted from a broadcasting satellite BS2 over the Eastern U.S. (strictly speaking, the satellites BS1 and BS2 are positioned along the Equator at longitudes corresponding to the Western U.S. and the Eastern U.S.). Also, the signals A3 and B3 are OFDM (Orthogonal Frequency Division Multiplex) signals and are transmitted from an antenna on the ground.
Therefore, since the signals A1, A2, B1, and B2 are satellite waves, and a diversity effect can be obtained by the satellites BS1 and BS2, a broadcast can be listened to over the entire U.S. Also, when there is a high-rise building, radio waves are sometimes blocked, but this is compensated for by the signals A3 and B3 of the terrestrial waves. Therefore, even when the receiving conditions of radio waves of a receiver mounted in a vehicle greatly change as the vehicle travels, it is possible to satisfactorily receive a broadcast.
In DARS, since the signals A1 to A3 and B1 to B3 are broadcast by frequency division in the above-described manner, a receiver therefor is constructed as shown in, for example, FIG. 7. In the following description, for brevity of explanation, as shown in FIG. 8A, the signals A1 and A2 are collectively denoted as A12, and the signals B1 and B2 are collectively denoted as B12.
More specifically, in FIG. 7, the signals A12, A3, B12, and B3 are received by an antenna 11, and the received signals A12 to B3 are supplied to a first mixer circuit 14 via a band-pass filter 12 and a high-frequency amplifier 13. Furthermore, a first local oscillation signal SLO is supplied from a first local oscillation circuit 15 to the first mixer circuit 14, whereby the signals A12 to B3 are frequency-converted into first intermediate frequency signals.
When the ensemble A is to be listened to (when the signals A1 to A3 are subjects to be received), as indicated by the solid line in FIG. 8A, the first local oscillation signal SLO is set to a predetermined frequency fL which is lower than those of the signals A12 and A3. Therefore, as shown in FIG. 8B, the signal A12 is frequency-converted into a first intermediate frequency signal SIF12 (at intermediate frequency fIF12), the signal A3 is frequency-converted into a first intermediate frequency signal SIF3 (at intermediate frequency fIF3), and the signals B12 and B3 are frequency-converted into first intermediate frequency signals SIF45 and SIF6, respectively.
When the image rejection characteristics are taken into consideration, the first intermediate frequencies fIF12 and fIF3 cannot be decreased too much, and since a frequency band of 2.3 GHz is used in a broadcast, the first intermediate frequencies fIF12 and fIF3 are set to 100 MHz or higher. For example, the following are set:                fIF12 is approximately 113 MHz, and fIF3 is approximately 116 MHz        
Also, when the ensemble B is to be listened to (when the signals B1 to B3 are subjects to be received), as indicated by the broken line in FIG. 8A, the first local oscillation signal SLO is set to a predetermined frequency fH which is higher than those of the signals B12 and B3. Therefore, as shown in FIG. 8C, the signal B12 is frequency-converted into a first intermediate frequency signal SIF12 (at intermediate frequency fIF12), the signal B3 is frequency-converted into a first intermediate frequency signal SIF3 (at intermediate frequency fIF3), and the signals A12 and A3 are frequency-converted into first intermediate frequency signals SIF45 and SIF6, respectively.
Therefore, when any one of the ensembles A and B is to be listened to, the intermediate frequency signals SIF12 to SIF6 are supplied to a band-pass filter 21L for a first intermediate frequency filter, whereby an intermediate frequency signal SIF12 is extracted. Then, this signal is supplied to a second mixer circuit 22L, a second local oscillation signal having a predetermined frequency is provided from a second local oscillation circuit 23, and this signal is supplied to the mixer circuit 22L, whereby the signal SIF12 is frequency-converted into a second intermediate frequency signal. Then, this signal is supplied to a demodulation circuit 25L via a variable gain amplifier 24L for AGC (Automatic Gain Control), whereby a digital audio signal of the target program is demodulated, and this signal is supplied to a selecting/combining circuit 26.
Also, the signals SIF12 to SIF6 from the first mixer circuit 14 is supplied to a band-pass filter 21H for a first intermediate frequency filter, whereby the intermediate frequency signal SIF3 is extracted. Then, this signal is supplied to a second mixer circuit 22H, and furthermore, a second local oscillation signal from the second local oscillation circuit 23 is supplied to the mixer circuit 22H, whereby the signal SIF3 is frequency-converted into a second intermediate frequency signal. Then, this signal is supplied to a demodulation circuit 25H via a variable gain amplifier 24H for AGC, whereby a digital audio signal of the target program is demodulated, and this signal is supplied to the selecting/combining circuit 26.
Then, in the selecting/combining circuit 26, the signal from the demodulation circuit 25L and the signal from the demodulation circuit 25H are selected or combined, and is output at an output terminal 27.
Therefore, as a result of switching the frequency of the first local oscillation signal SLO to a frequency fL or a frequency fH, a digital signal of the ensemble A or a digital signal of the ensemble B is output at the terminal 27.
Then, at that time, when the ensemble A is received, since the digital signal demodulated from the received signal A12 and the digital signal demodulated from the received signal A3 are selected or combined, and is taken out at the terminal 27, a digital signal having a small amount of error can be obtained regardless of the receiving conditions. Furthermore, also when the ensemble B is received, a digital signal having a small amount of error can be obtained regardless of the receiving conditions for the same reasons.
However, in the above-described receiver, when the ensemble is switched from the ensemble A to the ensemble B, it is necessary to change the frequency of the first local oscillation signal SLO from the frequency fL to the frequency fH. That is, as is also clear from FIGS. 8A to 8C, it is necessary to change the frequency of the first local oscillation signal SLO to a frequency larger than the occupied bandwidth 12.5 MHz of the services of the signals A1 to A3 and B1 to B3. Also, the same applies to a case in which the ensemble is changed from the ensemble B to the ensemble A.
The amount of change of this frequency is equal to or more than 10% of the frequencies fL and fH. Moreover, when the first local oscillation circuit 15 is formed by a PLL (Phase-Locked Loop), it is necessary to allow for some margin with respect to the range of change of the oscillation frequency of the VCO (Voltage Controlled Oscillator) of the PLL. For this reason, it is necessary to increase the range of change of the oscillation frequency of the VCO by making the resonance device of the VCO changeable. As a result, the construction becomes complex, and the phase noise characteristics of the local oscillation signal SLO deteriorate, causing the error rate of the digital signal to become worse.
Also, as long as the first local oscillation circuit 15 is formed by a PLL, it takes time to change the frequency, and the ensemble cannot be received during that change.
In addition, the first intermediate frequencies fIF12 and fIF3 are increased to 100 MHz or higher in the above-described manner, and as shown in FIGS. 8B and 8C, it is necessary for the filters 21L and 21H to extract the first intermediate frequency signals SIF12 and SIF3 from within the crowded signals. As a result, the filters 21L and 21H are formed by an SAW (Surface Acoustic Wave) filter. For this reason, the cost increases, and when the circuit is formed into an IC (integrated circuit), the SAW filter must be provided externally. Furthermore, this becomes an obstacle to the reduction in size of the receiver.
Also, when the demodulation of the demodulation circuits 25L and 25H is to be performed by a digital process, an intermediate frequency signal supplied to the demodulation circuits 25L and 25H must be formed into a frequency at which a digital process is possible. For this purpose, as is also shown in FIG. 7, for the receiving method, a double conversion method must be used, the construction becomes complex, and the number of parts is increased.